Expressing personal opinions:

In my opinion, …
Personally, I think that …
It seems to me that …
I must admit that …
I believe/suppose that …
I cannot deny that …
As I see it, …
As far as I’m concerned, …
I would argue that …
I’d like to point out that …
In my experience, …

✅Very strong opinions:

I am sure that …
I am convinced that …
I am certain that …
Example: I would argue that computers are rather beneficial for kids.

🔺Proving your arguments

to attest, …
to prove, …
this (fact) is attested/proven by …
this (fact) is evidenced by …
this (fact) is testified by …
this (fact) is endorsed/supported by…
this (evidence) establishes that …
Example: Consuming moderate amounts of sweets can be good for health. This is evidenced by a number of cases when people improved their health conditions by eating chocolate.

📍Expressing general point of view:

It is (generally) claimed that …
It is (generally) said that …
It is (generally) thought that …
It is (generally) considered that …
A common opinion is that …
A popular belief is that …
Example: It is considered that rigorous diets are very unhealthy.